Imaging apparatus and method for controlling flash emission

ABSTRACT

An imaging apparatus includes: flash driving means for driving a flash during a set preliminary emission period and a set main emission period; and an image sensor constituted by a plurality of photoelectric converting elements which are arranged in a matrix and capable of being accessed randomly. The charge accumulation initiation timing and the charge accumulation cessation timing of the photoelectric converting elements line are controlled line by line or pixel by pixel. The imaging apparatus forms images employing pixel signals output from the photoelectric converting elements, and determines main regions within the images. The preliminary emission period is set such that charge accumulation in photoelectric converting elements corresponding to pixels that constitute the main region is initiated prior to the preliminary emission period and ceased after the preliminary emission period. The flash driving means drives the flash during the set preliminary emission period.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to an imaging apparatus equipped with aflash photography function, and a method for controlling flash. Morespecifically, the present invention is related to an imaging apparatusthat obtains images with an imaging element which is capable ofcontrolling an initiation timing and a cessation timing of chargeaccumulation in pixel units, and a method, and optimizes flash emissionamounts utilizing the images, and a method for optimizing flash emissionamounts.

2. Description of the Related Art

Recently, imaging elements which are provided in digital cameras and thelike are transitioning form CCD image sensors to CMOS image sensors.CMOS image sensors can be manufactured by a process similar to that formanufacturing CMOS LSI's. Therefore, providing built in circuits otherthan image sensors into the same chip is facilitated, and CMOS imagesensors are suited for use as so called “systems on chips”. In addition,CMOS image sensors have advantages over CCD image sensors in that thereis less influence by noise (smearing) during signal transfer, and thenumber of power sources can be reduced.

On the other hand, CMOS image sensors cannot output signals from allpixels simultaneously due to their structures, unlike CCD image sensors.Therefore, signal readout is performed line by line or pixel by pixel.At this time, charge accumulation is initiated pixels from the point intime that signals are output therefrom. Therefore, the chargeaccumulation initiation timing and the charge accumulation completiontiming of each pixel differ according to the signal output timingthereof.

Meanwhile, there is a known method for controlling flash of digitalcameras, in which flash is emitted prior to actual photography(hereinafter, referred to as “preliminary emission”), and an imageobtained during the preliminary emission is utilized to optimize flashemission during actual photography (hereinafter, referred to as “mainemission”). This method is effective when the imaging element of thedigital camera is a CCD, but not suited for use in the case that theimaging element is a CMOS image sensor, in which the charge accumulationinitiation/completion timings are different among pixels. For thisreason, digital cameras which are equipped with CMOS image sensors arefurther provided with light adjusting sensors, and flash is controlledbased on data obtained by the light adjusting sensors.

Japanese Patent No. 3639734 discloses a method, in which flash emissionis controlled by utilizing images obtained by a CMOS image sensor. Thisdocument proposes a method in which the timing of preliminary emissionis adjusted at the factory which manufactures digital cameras, such thatpreliminary emission is performed during a period in which pixels withina specific block are accumulating charges. Then, the signals which areread out from the specific block are employed to control main emission.

The method disclosed in Japanese Patent No. 3639734 is that which readsout signals from the specific block at the preliminary emission timing,as described in paragraphs 0028 through 0030. In this method, it isnecessary to control readout operations depending on whether preliminaryemission is performed. However, there is a possibility that frequentswitching of control methods will lead to problems such as operatingerrors, which is not preferable. In addition, preliminary emission andoptimization of actual flash emission amounts are performed after ashutter release button is depressed, and therefore high speed processingis required. Therefore, complex processes that accompany frequentswitching of control methods is not preferable from the viewpoint ofprocessing time as well.

SUMMARY OF THE INVENTION

The present invention has been developed in view of the foregoingcircumstances. It is an object of the present invention to provide amethod for optimizing the amount of flash emission without complexreadout control even in cases that shutter speed is high (that is,exposure time is short), as a method for controlling flash emission inan imaging apparatus equipped with a CMOS sensor.

An imaging apparatus of the present invention comprises:

an image sensor constituted by a plurality of photoelectric convertingelements which are arranged in a matrix and capable of being accessedrandomly;

control means for controlling the charge accumulation initiation timingand the charge accumulation cessation timing of the photoelectricconverting elements line by line or pixel by pixel;

image forming means for forming images employing pixel signals which areoutput from the photoelectric converting elements;

main region determining means for determining portions of the imageswhich are formed by the image forming means as main regions;

flash driving means for driving a flash during a set preliminaryemission period and a set main emission period; and

period setting means for setting the preliminary emission period suchthat charge accumulation is initiated prior to the preliminary emissionperiod and charge accumulation is ceased after the preliminary emissionperiod in the photoelectric converting elements corresponding to pixelsthat constitute the determined main portions.

A method of the present invention is a method for controlling flashemission of an imaging apparatus that has a flash photography function,comprising the steps of:

controlling the charge accumulation initiation timing and the chargeaccumulation cessation timing of photoelectric converting elements of animage sensor constituted by a plurality of photoelectric convertingelements which are arranged in a matrix and capable of being accessedrandomly line by line or pixel by pixel;

forming images employing pixel signals which are output from thephotoelectric converting elements;

determining portions of the formed images as main regions;

setting a preliminary emission period such that charge accumulation isinitiated prior to the preliminary emission period and chargeaccumulation is ceased after the preliminary emission period in thephotoelectric converting elements corresponding to pixels thatconstitute the determined main portions; and

driving a flash during the set preliminary emission period.

In imaging elements such as CMOS image sensors, there are lines orpixels that cannot receive light reflected from subjects duringpreliminary emission, when the duration from charge accumulationinitiation to charge accumulation cessation is short, that is, when theshutter speed is high. In this respect, in the imaging apparatus and themethod for controlling flash of the present invention, the photoelectricconverting elements corresponding to the main region receive lightreflected from subjects during preliminary emission. Therefore, images(preliminary images) suited for calculating flash emission amounts to beemployed during main emission can be obtained even in cases that theshutter speed is high, and flash emission amounts to be employed duringmain emission can be set to appropriate values.

A configuration may be adopted, wherein:

the main region determining means detects predetermined patterns withinthe images formed by the image forming means, and determines regions inwhich the predetermined patterns are present as the main regions. Inthis case, it is preferable for the predetermined patterns to be imagesof faces or reference patterns which are utilized to detect movement ofsubjects.

A configuration may be adopted, wherein:

the main region determining means divides the images formed by the imageforming means into a plurality of blocks, calculates distances from eachblock to subjects within the images, and determines regions constitutedby at least one block having a distance to the subjects less than orequal to a predetermined threshold value as the main regions.

A configuration may be adopted, wherein:

the main region determining means divides the images formed by the imageforming means into a plurality of blocks, calculates the brightness ofsubjects within each block, and determines regions constituted by atleast one block having a subject brightness less than or equal to apredetermined threshold value as the main regions.

A configuration may be adopted, wherein:

the main region determining means receives input of region settingoperations by a user within the images formed by the image formingmeans, and determines regions set by the region setting operations asthe main regions.

It is preferable for the imaging apparatus to further comprise:

display control means for outputting the images formed by the imageforming means and markers that indicate the main regions determined bythe main region determining means to a predetermined screen. In thiscase, users can confirm the main regions on a monitor.

It is preferable for a configuration to be adopted, wherein:

the period setting means resets the preliminary emission period suchthat a period which is shifted for an amount of time corresponding to apredetermined threshold value from a reference period becomes thepreliminary emission period, in the case that the amount of temporalshift between the preliminary emission period, which has been set basedon the main region, and the reference period exceeds the predeterminedthreshold value. Alternatively, the period setting means may reset thepreliminary emission period such that a reference period becomes thepreliminary emission period, in such cases. Thereby, problems caused byset preliminary emission periods being greatly shifted from standardpreliminary emission periods can be resolved.

It is preferable for a configuration to be adopted, wherein:

the main region determining means operates selectively based on anoperating mode set in the imaging apparatus. That is, it is preferableto switch whether main regions are to be determined, and the method bywhich determinations are made according to operating modes.

It is preferable for the imaging apparatus of the present invention tofurther comprise:

means for recording the images formed by the image forming means into apredetermined recording medium, and for recording data that specifiesthe preliminary emission period as data attached to the images. In thiscase, users can read out the data from the recording medium along withthe images, and confirm the data. In addition, in cases that image filesare transferred to a personal computer and image processes areadministered thereon, the data can be utilized in the image processes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view that illustrates the outer appearance of adigital camera.

FIG. 2 is a rear view of the digital camera of FIG. 1.

FIG. 3 is a diagram that illustrates the inner structure of the digitalcamera of FIG. 1.

FIG. 4 is a chart that illustrates exposure operations and outputoperations of unit elements.

FIG. 5 illustrates an example in which an image is divided into 8×8block regions.

FIG. 6 is a flow chart that illustrates the steps of a process performedby a flash emission amount calculating section.

FIG. 7 is a diagram that illustrates an example of weightingcoefficients assigned to blocks of an image.

FIG. 8 is a diagram that illustrates the relationship between emissionperiods of a flash and the operation of a CMOS image sensor when thirdpreliminary images are obtained with a normal shutter speed.

FIG. 9 is a diagram that illustrates the relationship between emissionperiods of a flash and the operation of a CMOS image sensor when thirdpreliminary images are obtained with a high shutter speed.

FIG. 10 is a flow chart that illustrates the operations of the digitalcamera of FIG. 1 related to determining a flash emission amount (firstembodiment).

FIG. 11 is a diagram for explaining a method for determining apreliminary emission timing (first embodiment).

FIG. 12 is a diagram that illustrates an example of calculationparameters (weighting coefficients).

FIG. 13 is a flow chart that illustrates the operations of the digitalcamera of FIG. 1 related to determining a flash emission amount (secondembodiment).

FIG. 14 is a diagram for explaining a method for determining apreliminary emission timing (second embodiment).

FIG. 15 is a flow chart that illustrates the operations of the digitalcamera of FIG. 1 related to determining a flash emission amount (thirdembodiment).

FIG. 16 is a diagram for explaining a method for determining apreliminary emission timing (third embodiment).

FIG. 17 is a flowchart that illustrates the operations of the digitalcamera of FIG. 1 related to determining a flash emission amount (fourthembodiment).

FIG. 18 is a diagram that illustrates an example of an area guidedisplay.

FIG. 19A is a diagram for explaining a method for determining apreliminary emission timing (fourth embodiment),

FIG. 19B is a diagram for explaining a method for determining apreliminary emission timing (fourth embodiment),

FIG. 20A is a flow chart that illustrates the steps of a process whichis a modification of the operations of FIG. 17.

FIG. 20B is a flow chart that illustrates the steps of a process whichis another modification of the operations of FIG. 17.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an SLR digital camera equipped with a flash photographyfunction and a method for controlling the flash of the digital camerawill be described as embodiments of the imaging apparatus and thecontrol method of the present invention.

First Embodiment

FIG. 1 is a perspective view that illustrates the outer appearance of adigital camera 1. As illustrated in FIG. 1, an imaging lens 2 isprovided on the front surface of the digital camera 1. A shutter releasebutton 3; a flash 4; a hot shoe 5, which is a mounting port forperipheral equipment; and setting/operating buttons 6 that include amode dial 61 for performing various setting operations; a command dial62; and a flash button 63 are provided on the upper surface of thedigital camera 1.

The shutter release button 3 is of a structure that can command twotypes of operations by being depressed in two steps. For example, duringa photography operation that utilizes an AE (Automatic Exposure)function and an AF (Automatic Focus) function, the digital camera 1 setsan optimal exposure and an optimal focal point when the shutter releasebutton 3 is depressed lightly (also referred as “half depression”). Ifthe shutter release button 3 is depressed strongly (also referred to as“full depression”) in this state, the digital camera initiates exposurewith set conditions, then records image data obtained by the exposurecorresponding to a single frame into a memory card.

The flash 4 is housed in a flash housing section 7. The flash performsemission operations accompanying second step depressions (fulldepressions) of the shutter release button 3. The flash housing section7 opens upward from the camera automatically or by manual operation.FIG. 1 illustrates the flash housing section 7 in an open state.

The emission operation of the flash is determined by photography modesand flash modes which are set by a user. The photography modes include:an AUTO mode, in which all settings involved with photography operationsare automatically set by the digital camera 1; a manual mode, in whichall settings involved with photography operations are manually set bythe user; a program auto mode; a shutter prioritizing auto mode; anaperture prioritizing auto mode; a preliminary reduced mode; a naturalphoto mode; and modes set for photography scenes, such as portraits,scenery, and night views. The photography mode can be set by operatingthe mode dial 61. The flash modes include: an automatic flash mode, inwhich whether it is necessary to emit flash is judged from thebrightness of the surroundings; and a forced emission mode, in whichflash is emitted regardless of the brightness of the surroundings. Theflash mode can be set by depressing the flash button 63, and performingoperations on a setting screen which is displayed on a monitor to bedescribed later.

An external flash may be mounted onto the hot shoe 5 of the digitalcamera 1 in a state in which the flash housing section 7 is closed. Theexternal flash is mechanically and electrically connected to the digitalcamera by being mounted onto the hot shoe 5. Thereby, the external flashperforms emission operations accompanying depressions of the shutterrelease button 3 according to set operating modes, in the same manner asthe built in flash 4. The present invention is applicable to cases inwhich an imaging apparatus utilizes an external flash as well as casesin which an imaging apparatus utilizes a built in flash.

FIG. 2 is a rear view of the digital camera 1. As illustrated in FIG. 2,an LCD (Liquid Crystal Display) monitor 9, a cruciform key 64 which isemployed to select options during setting operations, and varioussetting operation buttons, such as a zoom setting button, are providedon the rear surface of the digital camera 1. A slot cover 10 provided onthe side surface of the digital camera 1 is openable and closable, and amemory card slot is provided within the slot cover 10. Images which arephotographed by the digital camera 1 are recorded into a recordingmedium such as an xD picture card loaded in the memory card slot.

FIG. 3 is a diagram that illustrates the inner structure of the digitalcamera 1. As illustrated in FIG. 3, the digital camera 1 is equippedwith a CPU 42 and an internal memory 48, in which various controlprograms for controlling photography operations and various settingvalues are stored. In addition, the digital camera 1 is equipped with anoperating system control section 43, into which data is input from anoperating system including the shutter release button 3 and the varioussetting/operating buttons 6. The CPU 42 sends and receives signals withthe components of the digital camera 1 either directly or via a systembus 54. The CPU 42 controls updates of set values stored in the internalmemory 48 according to operation data input from the operating systemcontrol section 43 and control programs stored in the internal memory48. The CPU 42 also functions to control the operations of variouscomponents of the digital camera 1 to be described hereinafter.

The digital camera 1 is equipped with: imaging lenses 2; an aperture 31;a shutter 32; a CMOS image sensor 33; and drive sections for driving andcontrolling these components, as an imaging system 30. The imaginglenses 2 are constituted by a focusing lens 21 and a zoom lens 22. Thefocusing lens 21 and the zoom lens 22 are driven by a focus lens drivingsection 34 and a zoom lens driving section 35, which are respectivelyconstituted by a motor and a motor driver, so as to be movable in thedirection of the optical axes thereof. The focusing lens driving section34 drives the focusing lens 21 based on focus driving amount data outputfrom an AF processing section 45 to be described later. The zoom lensdriving section 35 controls the driving of the zoom lens 22 based onoperation amount data of a zoom lever.

The aperture 31 is driven by an aperture driving section 36 constitutedby a motor and a motor driver. The aperture driving section 36 adjustshow much the aperture 31 is opened based on aperture value data outputfrom an AE/AWB processing section 46 to be described later.

The shutter 32 is a mechanical shutter, and is driven by a shutterdriving section 37 constituted by a motor and a motor driver. Theshutter driving section 37 controls the opening and closing of theshutter 32 according to a signal which is generated when the shutterrelease button 3 is depressed, and also according to shutter speed dataoutput from the AE/AWB processing section 46.

The CMOS image sensor 33, which is an imaging element, is providedtoward the rear of the optical system described above. The CMOS imagesensor 33 has a light receiving surface, in which a plurality ofphotoelectric converting elements are arranged in a matrix. A microlensarray for focusing light onto each pixel, and a color filter array, inwhich R, G, and B filters are arranged in a regular manner, are providedin front of the photoelectric converting surface. Light which has passedthrough the optical system is focused onto the light receiving surface,and photoelectrically converted such that charges are accumulated ineach photoelectric converting element.

The accumulated charges are amplified by an amplifying element which isprovided behind each photoelectric converting element. The charges areoutput as analog image signals one line at a time, synchronized withvertical transfer clock signals and horizontal transfer clock signalsprovided by a CMOS driving section 38.

In the following description, each element constituted by aphotoelectric converting element and an amplifying element correspondingto a single pixel will be referred to as a unit element. The topmostline of the light receiving surface will be referred to as line 1, andsubsequent lines will be referred to as line 2, line 3, . . . line N (Nis the total number of lines).

FIG. 4 is a chart that illustrates the operations of unit elements thatconstitute each line of the CMOS image sensor 33. The horizontal axis inthe chart of FIG. 4 represents time. As illustrated in FIG. 4, whencharges which are accumulated in the unit elements are output in orderfrom line 1 and the amount of time required to output charges for asingle line is To, the time required to output a single frame becomesTo·N.

In CCD's, which were conventionally used as imaging elements, all unitelements initiate a next exposure operation (accumulation of charges)simultaneously after output of a single frame is complete. In contrast,each unit element of a CMOS image sensor initiates accumulation ofcharges immediately after charges accumulated therein are output. Forthis reason, it is possible to control the timing at which each line oreach unit element of a CMOS image sensor initiates accumulation ofcharges, by controlling the timing at which charges are output. If theoutput timings are controlled to read out signals in units of lines, theexposure periods of unit elements that constitute each line are shiftedby an amount of time To, as illustrated in FIG. 4.

The amount of time To required to output charges is determined by thespecifications (performance) of each device, and is stored in theinternal memory 48. Meanwhile, the charge accumulation period, that is,an amount of exposure time Te, is controlled by electronic shutter drivesignals which are output by the CMOS driving section 38. The CMOSdriving section 38 determines the exposure time Te based on a shutterspeed obtained by the AE/AWB processing section 46 or based on a shutterspeed set by the setting/operation buttons 6. Clock control is exertedsuch that each unit element outputs charges when the exposure time Teelapses.

A description will be given with reference to FIG. 3. Analog signalswhich are output from the CMOS image sensor 33 are input into an analogsignal processing section 39. The analog signal processing section 39 isconstituted by: a correlated double sampling (CDS) circuit for removingnoise from the analog image signals; an automatic gain controller (AGC)for adjusting the gain for the analog image signals; and an A/Dconverter (ADC) for converting the analog image signals to digital imagedata. The analog signals input into the analog signal processing section39 are converted into CCD-RAW data having RGB density values for eachpixel by the above circuits.

A timing generator 40 generates timing signals. The timing signals areinput to the shutter driving section 37, the CMOS driving section 38,and the analog signal processing section 39, to synchronize theoperation of the shutter release button 3, the opening and closing ofthe shutter 32, readout of charges from each line or each pixel of theCMOS image sensor 33, and the processing by the analog signal processingsection 39.

An image input controller 44 organizes the digital data input from theanalog signal processing section 39 into frame units, and writes thedigital data in a frame memory 49. That is, in the present embodiment,the analog signal processing section 39 and the image input controller44 function as an image forming means.

The frame memory 49 is a memory used as workspace for various types ofdigital image processes (signal processing) on the image data, whichwill be described later, and comprises an SDRAM (Synchronous DynamicRandom Access Memory) that carries out data transfer in synchronizationwith a bus clock signal of a predetermined period, for example.

A display control section 50 functions to display the image data storedin the frame memory 49 as a through the lens image on the monitor 9, andto display image data stored in a recording medium 8 in a playback mode.Note that the through the lens image is continuously obtained by theCMOS image sensor 33 at predetermined intervals when a photography modeis selected.

The AF processing section 45 and the AE/AWB processing section 46determine photography conditions based on a first preliminary image. Thefirst preliminary image is obtained by the CMOS image sensor 33 when theCPU 42 detects a half depression operation of the shutter release button3, and is an image which is stored in the frame memory 49 via the analogsignal processing section 39 and the image input controller 44.

The AF processing section 45 detects a focusing position based on thefirst preliminary image, and outputs focusing drive amount data (AFprocessing). A passive method, in which a characteristic that a focusevaluation value (contrast value) of images increases in a focused stateis utilized, may be applied to detect the focusing position.

The AE/AWB processing section 46 measures the luminance of subjectsbased on the first preliminary image, and determines exposure conditionssuch as an ISO sensitivity, an aperture value, an shutter speed and thelike, based on the luminance. The AE/AWB processing section 46 thenoutputs ISO sensitivity data, aperture value data, and shutter speeddata as exposure setting values (AE processing). At the same time, theAE/AWB processing section 46 automatically adjusts white balance duringphotography (AWB processing). Note that the exposure and white balancemay be set by manual operations input by a photographer, if thephotography mode is set to manual mode. In addition, even in the casethat the exposure and white balance are set automatically, aphotographer may input commands through the operating system to manuallyadjust the exposure and white balance.

An image processing section 47 administers image quality enhancementprocesses such as gradation correction, sharpness correction, and colorcorrection on image data of a final image. The image processing section47 also administers YC processes to convert the CCD-RAW data into YCdata comprising Y data as a luminance signal, Cb data as a blue colordifference signal, and Cr data as a red color difference signal. Thefinal image is an image based on the image data stored in the framememory 49 via the analog signal processing section 39 and the imageinput controller 44 after input of the analog image data from the CMOSimage sensor 33, in response to a full depression of the shutter button3. The maximum number of pixels of the final image is determined by thenumber of the pixels of the CCD 58. However, the number of pixels to berecorded can be changed by the user, by setting the image quality tofine or normal, for example. The number of pixels of the through thelens image and the preliminary image may be less than that of the finalimage, and may be 1/16 that of the final image, for example.

A compression/decompression section 51 carries out compression processesto a format such as JPEG on the image data, which has been subjected tothe image enhancement processes and the like by the image processingsection 47, and generates an image file. Accompanying information isadded as tags to the image file, based on the Exif format. Thecompression/decompression section 51 also reads compressed images filesfrom the recording medium 8 in the playback mode, and administersdecompression processes thereon. Image data, on which the decompressionprocesses have been administered, are displayed on the monitor 9.

A media control section 52 carries out image-file reading and writingfrom and to the recording medium 8.

A face detecting section 55 detects patterns that represent faces(hereinafter, referred to as “facial patterns”) from within through thelens images obtained prior to operation of the shutter release buttonand from within preliminary images obtained after operation of theshutter release button. In detection from within through the lensimages, a face which is detected first is set as a reference pattern,and movement of the face is detected by obtaining differences amongsubsequently detected facial patterns. Alternatively, a pattern of anentire person that includes the face which is detected first is set as areference pattern, and movement of the person is detected.

A distance measuring section 56 divides images into a plurality ofblocks, and obtains distances to subjects from each block. Thecalculation of distances is performed employing a plurality ofpreliminary images having different focusing positions which areobtained during the AF process. In these preliminary images, blockswhich are in focus have the highest image contrast. Therefore, thedistances to the subjects within these blocks can be obtained asdistances to focusing positions.

The flash 4 is driven by the flash driving section 41 to emits lightwhen the CPU 42 detects a full depression operation of the shutterrelease button 3 in a state that the photography mode or the flash modeis set to a mode in which flash is to be emitted. Flash emission isperformed a total of two times, once for preliminary emission and oncefor main emission. Flash emission is controlled by the flash drivingsection 41, the CPU 42, and a flash emission amount calculating section53.

The flash emission amount calculating section 53 determines the amountof flash to be emitted during main emission, based on a secondpreliminary image and a third preliminary image. The second preliminaryimage is obtained by the CMOS image sensor 33 prior to preliminaryemission of flash when the CPU 42 detects a full depression operation ofthe shutter release button 3, and is an image represented by image datawhich is stored in the frame memory 49 via the analog signal processingsection 39 and the image input controller 44. The third preliminaryimage is obtained by the CMOS image sensor 33 during the preliminaryemission of flash, and is an image represented by image data which isstored in the frame memory 49.

In the present embodiment, the flash emission amount calculating section53 determines the amount of flash to be emitted during main emissionbased on the brightness of the central portion of an image. The flashemission amount calculating section 53 reads out the second preliminaryimage from the frame memory 49, and divides the second preliminary imageinto a plurality of block regions.

FIG. 5 illustrates an example in which an image is divided into 8×8block regions.

FIG. 6 is a flow chart that illustrates the steps of a process performedby the flash emission amount calculating section 53. As illustrated inFIG. 6, the flash emission amount calculating section 53 obtains averagebrightness values for each block within a second preliminary image (stepS101). Specifically, the second preliminary image, which is an RGBimage, is converted to a YCC image, to obtain brightness values for eachpixel. Then, the average brightness values of pixels in each block arecalculated. Average brightness values are obtained for each block withina third preliminary image in the same manner (step S102). Then,differences among the average brightness values obtained in step S102and the average brightness values obtained in step S101 are calculated(step S103).

Next, the flash emission amount calculating section 53 sets weights forthe differences for each block which were calculated in step S103, andcalculates a weighted average (step S104). Thereby, a reflectedbrightness E1 during preliminary emission is obtained. The weights areset higher for blocks arranged at positions closer to the center of theimage, as illustrated in the example of FIG. 7. That is, the reflectedbrightnesses of subjects arranged at the center are caused to be morestrongly reflected in the calculation results.

The flash emission amount calculating section 53 obtains a magnificationrate M, which is the magnification rate of the flash emission amountduring main emission with respect to that during the preliminaryemission, according to Formula (1) below (step S105).

M=(E2−E3)/E1  (1)

wherein E2 is a target brightness of an image to be obtained by actualphotography, and E3 is the average brightness of the entire secondpreliminary image, which is obtained without emitting flash.

Then, data that represents the magnification rate M, or data thatrepresents the flash emission amount to be utilized for main emission,calculated by multiplying the flash emission amount during preliminaryemission by M, is output as flash emission amount data (step S106).

The flash emission amount data output from the flash emission amountcalculating section 53 is provided to the flash driving section 41 viathe CPU 42. The flash driving section 41 adjusts the bulb voltage andemission time of the flash 4 such that the flash emission amount duringmain emission becomes that indicated by the flash emission amount data,and drives the flash 4.

Note that various other methods for calculating the flash emissionamount during main emission based on an image obtained without emittingflash and an image obtained during preliminary emission may be employed.For example, there is a known method for determining a flash emissionamount by performing calculations with weighting placed on regions atwhich faces are detected, in digital cameras that have a function ofdetecting faces within images. The flash emission amount calculatingsection 53 may adopt such a known method to calculate flash emissionamounts, and the process described above is merely an illustrativeexample.

Hereinafter, operations related to controlling the flash of the digitalcamera 1 will be described further. FIG. 8 and FIG. 9 are diagrams thatillustrate relationships between emission periods of the flash 4 and theoperation of the CMOS image sensor 33 when third preliminary images areobtained. FIG. 8 illustrates the relationship in the case that a normal(not high) shutter speed is set, and FIG. 9 illustrates the relationshipin the case that a high shutter speed is set.

As illustrated in FIG. 8, the exposure time Te of the unit elementsillustrated in FIG. 4 becomes sufficiently long compared to the outputtime To at a normal shutter speed setting. In this case, a comparativelylong period during which all of the unit elements are accumulatingcharges is present. Accordingly, if preliminary emission is performed ata midpoint of a period beginning at a charge accumulation initiationtime of line 1 and a charge accumulation completion time of line N,light reflected during the preliminary emission period is received byunit elements that constitute all lines of the CMOS image sensor 33. Inother words, all of the unit elements that constitute the CMOS imagesensor 33 initiate charge accumulation prior to the preliminary emissionperiod and cease charge accumulation after the preliminary emissionperiod. The third preliminary image is formed from pixel signals outputfrom these unit elements.

On the other hand, when the shutter speed is high, that is, the exposuretime Te is short, a period during which all of the unit elements areaccumulating charges is not present, or extremely short, as illustratedin FIG. 9. For this reason, only unit pixels of a limited number oflines output effective pixel signals which are generated by receivingreflected light during the preliminary emission period. In addition, asillustrated in FIG. 9, the image ranges from which effective pixelsignals can be obtained differ in the case that preliminary emission isinitiated at a timing A, which is the approximate midpoint of a chargeaccumulation period of line L1 and the case that preliminary emission isinitiated at a timing B, which is the approximate midpoint of a chargeaccumulation period of line L2.

In the example illustrated in FIG. 9, if preliminary emission isinitiated at timing A, effective pixel signals can be obtained within aregion that represents a face within the image. However, if preliminaryemission is initiated at timing B, effective pixel signals cannot beobtained within a region that represents a face within the image.Therefore, in the present embodiment, the preliminary emission period isadjusted such that preliminary emission is initiated at timing A, by thesteps to be described below.

FIG. 10 is a flow chart that illustrates the operations of the digitalcamera 1 related to determining the flash emission amount to be employedduring main emission. When a half depression operation of the shutterrelease button 3 is detected by the CPU 42 (step S201), a firstpreliminary image is obtained by commands issued by the CPU 42 andstored in the frame memory 49 (step S202). The first preliminary imageis utilized by the AE/AWB processing 46 as described previously, and anexposure time Te is set by an AE process (step S203).

Meanwhile, the first preliminary image stored in the frame memory 49 isalso read out by the face detecting section 55. The face detectingsection 55 searches for facial patterns within the first preliminaryimage, and in the case that a facial pattern is detected, main regiondata that indicates a region in which the face pattern is present isoutput (step S204). That is, in the present embodiment, the facedetecting section 55 functions as the main region determining means. Inaddition, line numbers and row numbers (La, Ra) and (Lb, Rb) thatrepresent the opposing corners of a rectangular region that includes thefacial pattern are output as data that specifies the rectangular regionthat includes the facial pattern, as illustrated in FIG. 11.

When the CPU 42 detects a full depression operation of the shutterrelease button 3 (step S205), the flash emission amount calculatingsection 53 determines the timing at which preliminary emission isinitiated to match the position of the detected face (step S206). Here,the timing at which preliminary emission is initiated is represented byan amount of time Tf between initiation of charge accumulation by unitelements of line 1 and initiation of preliminary emission. Note that thepreliminary emission time Tp is set in advance, and the preliminaryemission completion timing is defined by an amount of time (Tf+Tp). Thepreliminary emission period is set in this manner.

The flash emission amount calculating section 53 obtains a line thatincludes the central pixel within the face pattern, based on the mainregion data output by the face detecting section 55. In the exampleillustrated in FIG. 11, the line number L3 that includes the centralpixel within the facial pattern can be obtained by Formula (2).

L3=(La+Lb)/2  (2)

Thereafter, the amount of time between initiation of charge accumulationby unit elements of line 1 to the midpoint of a charge accumulationperiod of Line 3 is calculated, based on the set exposure time Te andthe output time To stored in the internal memory 48. That is, a timefrom line L1 to the midpoint of the charge accumulation period of LineL3 is determined as the preliminary emission initiation timing. Then,the calculated time is stored as time Tf that represents the initiationtiming of preliminary emission in the internal memory 48.

Further, the flash emission amount calculating section 53 determines thelines to be utilized in calculations based on the set exposure time Te,the output time To stored in the internal memory 48, the time Tf thatrepresents the initiation timing of preliminary emission, and thepreliminary emission time Tp (step S207).

For example, the range of lines to be utilized to calculate the flashemission amount is determined by Formula (3) below.

Lmin=(Tf+Tp−Te)/To+1,Lmax=Tf/To+1  (3)

wherein Lmin is the line having the smallest line number and Lmax is theline having the largest line number within the range of lines. Note thatin the case that Lmin<1, Lmin is designated to be 1, and in the casethat Lmax>N, Lmax is designated to be N. The line numbers that specifythe determined range are stored in the internal memory 48.

After the calculation area is determined, a second preliminary image isobtained according to commands output by the CPU 42, and stored in theframe memory 49 (step S208). Further, preliminary emission is initiatedby the flash driving section 41 at the timing indicated by the time Tf,and a third preliminary image obtained at this time is stored in theframe memory (step S209).

Next, the flash emission amount calculating section 53 reads out thesecond preliminary image and the third preliminary image from the framememory 49, and extracts effective pixel signals, that is, pixel signalswithin the calculation area, from the preliminary images. Thereafter,the process of the flow chart of FIG. 6 is executed with respect to onlythe extracted pixel signals, to calculate the flash emission amount tobe employed during main emission (step S210). However, in the processfor obtaining the weighted average at step S104 of FIG. 6, adjustedweighting coefficients illustrated in FIG. 12 are employed instead ofthe weighting coefficients illustrated in FIG. 7. That is, it ispreferable for the weighted average to be calculated with higherweighting at block group B2 that corresponds to the region indicated bythe main region data, from among the block group B1 that corresponds tothe effective pixel signals.

When the flash emission amount is determined, main emission is performedaccording to commands output from the CPU 42. and a final image obtainedat this time is stored in the frame memory 49 (step S211). The finalimage stored in the frame memory 49 is recorded in the recording medium8 via the media control section (step S212). The final image iscompressed by the compression/decompression section 51 as necessary, andrecorded as an image file of a predetermined format (an Exif file, forexample).

In the present embodiment, the time Tf that represents the initiationtiming of preliminary emission, line numbers Lmin and Lmax that specifythe range to be employed in calculations, and data regarding parametersto be employed in calculations, such as adjusted weighting parameters,are recorded in a predetermined region of image files as data attachedto final images. For example, the data may be recorded in a regiondefined by a manufacturer (MakerNote) within a tag region of an Exiffile.

As described above, in the digital camera 1 of the present embodiment,the initiation timing of preliminary emission is set according to theposition of a detected face. Therefore, even if a portion of theelements cannot receive reflected light during the preliminary emissionperiod due to a high shutter speed, pixel signals suited for use incalculations can be obtained in the periphery of the face. Accordingly,the flash emission amount to be employed during main emission can be setto an appropriate value.

Note that the initiation timing of preliminary emission may bedetermined based on the position of the reference pattern which isutilized by the face detecting section 55 to detect movement, instead ofthe position of the facial pattern. That is, the detected pattern is notlimited to faces, but may be the entire body of a person, a portion ofthe body of a person, or a subject other than a human.

Second Embodiment

Next, an embodiment in which the preliminary emission period iscontrolled based on distances to subjects will be described. Note thatthe structures and processes other than those related to control of theflash 4 are the same as those of the digital camera 1 of the firstembodiment, and therefore, detailed descriptions thereof will beomitted. In the present embodiment, the distance measuring section 56 ofthe digital camera 1 functions as the main region determining means.

FIG. 13 is a flow chart that illustrates the steps of an operation todetermine a flash emission amount to be employed during main emission bythe digital camera 1 of the second embodiment. When a half depressionoperation of the shutter release button 3 is detected by the CPU 42(step S301), a first preliminary image is obtained by commands issued bythe CPU 42 and stored in the frame memory 49 (step S302). The firstpreliminary image is utilized by the AE/AWB processing 46 as describedpreviously, and an exposure time Te is set by an AE process (step S303).

Meanwhile, the first preliminary image stored in the frame memory 49 isalso read out by the distance measuring section 56. The distancemeasuring section 56 obtains distances to subjects for each block, andoutputs main region data that indicates a block having the closestdistance to a subject (step S304). FIG. 14 is a diagram that illustratesa block defined within an image by the process performed by the distancemeasuring section 56. Block group B3 surrounded by the bold frameindicates blocks which have been judged to have the shortest distancesto the subject by the distance measuring section 56.

When the CPU 42 detects a full depression operation of the shutterrelease button 3 (step S305), the flash emission amount calculatingsection 53 obtains a line that includes the central pixel within theblock group B3 indicated by the main region data. Thereafter, the amountof time between initiation of charge accumulation by unit elements ofline 1 to the midpoint of a charge accumulation period of line L4 iscalculated, based on the set exposure time Te and the output time Tostored in the internal memory 48. Then, the calculated time is stored astime Tf that represents the initiation timing of preliminary emission inthe internal memory 48. The preliminary emission period is set in thismanner (step S306).

Further, the flash emission amount calculating section 53 determines acalculation area based on the set exposure time Te, the output time Tostored in the internal memory 48, the time Tf that represents theinitiation timing of preliminary emission, and the preliminary emissiontime Tp, by the same process as that of step S207 of the firstembodiment (step S307). Thereafter, steps S308 through 5312, which arethe same as steps S208 through S212 of the first embodiment, areexecuted.

The digital camera 1 of the second embodiment sets the preliminaryemission period according to the closest subject. Therefore, even if aportion of the elements cannot receive reflected light during thepreliminary emission period due to a high shutter speed, pixel signalssuited for use in calculations can be obtained regarding the closestsubject. Accordingly, the flash emission amount to be employed duringmain emission can be set to an appropriate value.

Third Embodiment

Next, an embodiment in which the preliminary emission period iscontrolled based on the brightness of subjects will be described. Notethat the structures and processes other than those related to control ofthe flash 4 are the same as those of the digital camera 1 of the firstembodiment, and therefore, detailed descriptions thereof will beomitted. In the present embodiment, the AE/AWB processing section 47 ofthe digital camera 1 functions as the main region determining means.

FIG. 15 is a flow chart that illustrates the steps of an operation todetermine a flash emission amount to be employed during main emission bythe digital camera 1 of the third embodiment. When a half depressionoperation of the shutter release button 3 is detected by the CPU 42(step S401), a first preliminary image is obtained by commands issued bythe CPU 42 and stored in the frame memory 49 (step S402). The firstpreliminary image is utilized by the AE/AWB processing 46 as describedpreviously, and an exposure time Te is set by an AE process (step S403).

When the CPU 42 detects a full depression operation of the shutterrelease button 3 (step S404), a through the lens image at the point intime when the shutter release button 3 was fully depressed is obtainedas a second preliminary image, and stored in the frame memory 49 (stepS405). Here, the AE/AWB processing section 46 measures subjectbrightnesses within the second preliminary image, and obtains brightnessdata for each block within the image (step 5406). Then, main region datathat indicates one or a plurality of blocks having brightnesses lessthan a threshold value is output. FIG. 16 is a diagram that illustratesa block defined within an image by the process performed by the AE/AWBprocessing section. Block group B4 surrounded by the bold frameindicates blocks which have been judged to have brightnesses less thanthe threshold value.

The flash emission amount calculating section 53 obtains a line L5 thatincludes the central pixel within the blocks indicated by the mainregion data. Thereafter, the amount of time between initiation of chargeaccumulation by unit elements of line 1 to the midpoint of a chargeaccumulation period of line L5 is calculated, based on the set exposuretime Te and the output time To stored in the internal memory 48. Then,the calculated time is stored as time Tf that represents the initiationtiming of preliminary emission in the internal memory 48. Thepreliminary emission period is set in this manner (step S407).

Further, the flash emission amount calculating section 53 determines acalculation area based on the set exposure time Te, the output time Tostored in the internal memory 48, the time Tf that represents theinitiation timing of preliminary emission, and the preliminary emissiontime Tp, by the same process as that of step S207 of the firstembodiment (step S408). Thereafter, steps S409 through S413, which arethe same as steps S208 through S212 of the first embodiment, areexecuted.

The digital camera 1 of the third embodiment sets the timing ofpreliminary emission according to a region having a normal brightness,excluding particularly bright regions (such as overexposed portions andclear skies). Therefore, even if a portion of the elements cannotreceive reflected light during the preliminary emission period due to ahigh shutter speed, pixel signals suited for use in calculations can beobtained regarding the main region. Accordingly, the flash emissionamount to be employed during main emission can be set to an appropriatevalue.

Fourth Embodiment

Next, an embodiment in which the preliminary emission period iscontrolled based on image regions set by a user will be described. Notethat the structures and processes other than those related to control ofthe flash 4 are the same as those of the digital camera 1 of the firstembodiment, and therefore, detailed descriptions thereof will beomitted. In the present embodiment, the image processing section 47 ofthe digital camera 1 functions as the main region determining means.

FIG. 17 is a flow chart that illustrates the steps of an operation todetermine a flash emission amount to be employed during main emission bythe digital camera 1 of the fourth embodiment. The digital camera 1sequentially stores images obtained by the imaging system 30 in theframe memory 49 temporarily prior to operation of the shutter releasebutton 3, and displays the images as through the lens images on themonitor 9 via the display control section 50. During this time, theimage processing section 47 reads out the images stored in the framememory 49, combines images of frames (hereinafter, referred to as “areaguides”) that function as markers to indicate main regions within theimages with the images, and updates the images in the frame memory 49with the combined images. Thereby, images which are combined with thearea guides 11 are displayed on the monitor 9 as through the lensimages, as illustrated in FIG. 18 (step S501).

A user changes the region indicated by the area guide 11, by moving,enlarging, or reducing the area guide 11, and performs predeterminedconfirmation operations, to set a desired region. When area changingoperations are detected by the CPU 42 (step S502), the image processingsection 47 recombines the through the lens image and the image of thearea guide 11. When the CPU 42 detects an area setting (confirming)operation (step S502), the image processing section 47 stores data thatindicates the set region within the internal memory 48. Thereby, thearea specified by the user is set as a main region (step S503).

Thereafter, when a half depression operation of the shutter releasebutton 3 is detected by the CPU 42 (step S504), a first preliminaryimage is obtained by commands issued by the CPU 42 and stored in theframe memory 49 (step S505). The first preliminary image is utilized bythe AE/AWB processing 46 as described previously, and an exposure timeTe is set by an AE process (step S506).

When the CPU 42 detects a full depression operation of the shutterrelease button 3 (step S507), the flash emission amount calculatingsection 53 obtains a line L6 or a line L7 that includes the centralpixel within the area set by the user operations, as illustrated in FIG.19A and FIG. 19B. Thereafter, the amount of time between initiation ofcharge accumulation by unit elements of line 1 to the midpoint of acharge accumulation period of line L6 or line L7 is calculated, based onthe set exposure time Te and the output time To stored in the internalmemory 48. Then, the calculated time is stored as time Tf thatrepresents the initiation timing of preliminary emission in the internalmemory 48. The preliminary emission period is set in this manner (stepS508).

Further, the flash emission amount calculating section 53 determineslines to be utilized in calculations (calculation area) based on the setexposure time Te, the output time To stored in the internal memory 48,the time Tf that represents the initiation timing of preliminaryemission, and the preliminary emission time Tp (step S509). In the casethat the area illustrated in FIG. 19A is set, the determination of thecalculation area is performed by the same process as that of step S207of the first embodiment. In the case that an area that exceeds the rangeof the image is set as illustrated in FIG. 19B, a range from the upperedge of the image to the lower edge of the set area is determined to bethe calculation area. Thereafter, steps S510 through S514, which are thesame as steps S208 through S212 of the first embodiment, are executed.

The digital camera 1 of the fourth embodiment sets the timing ofpreliminary emission according to an area set by a user. Therefore, evenif a portion of the elements cannot receive reflected light during thepreliminary emission period due to a high shutter speed, pixel signalssuited for use in calculations can be obtained regarding a subjectimportant to the user. Accordingly, the flash emission amount to beemployed during main emission can be set to an appropriate value. Forexample in the images illustrated in FIG. 19A and FIG. 19B, generally,digital cameras perform control of flash with the person in theforeground of the image as a main subject. However, the digital cameraof the present embodiment is capable of controlling the flash with theperson in the background of the image as a main subject, if the userdesires. That is, user intentions can be reflected in the control offlash.

Note that in the case that an area that exceeds the range of the imageis set, as illustrated in FIG. 19B, if the preliminary emission periodis determined based on the main region set by the user, there are casesthat the preliminary flash period shifts greatly from a standardpreliminary emission period (reference period). Here, the referenceperiod is a period having the midpoint of a period beginning at a chargeaccumulation initiation time of line 1 and a charge accumulationcompletion time of line N as the preliminary emission initiation timing.If the preliminary emission period shifts greatly from the referenceperiod, the number of lines that can be utilized for calculationsdecreases, and there is a possibility that problems will occur in thecalculations.

Accordingly, it is desirable for preliminary emission periods determinedbased on areas specified by users to be adjusted and reset, in casesthat temporal shifts between the preliminary emission periods and thereference period exceed a predetermined threshold value.

FIG. 20A and FIG. 20B illustrate processes for adjusting preliminaryemission periods in cases that the preliminary emission period set instep S508 is shifted greatly from the reference period, having themidpoint of a period beginning at a charge accumulation initiation timeof line 1 and a charge accumulation completion time of line N as thepreliminary emission initiation timing.

FIG. 20A illustrates a process by which the period preliminary emissionperiod is reset such that a period which is shifted for an amount oftime corresponding to a predetermined threshold value from a referenceperiod becomes the preliminary emission period. Tmin is a thresholdvalue which is set in advance by subtracting an allowable amount oftemporal shift from a time that represents the midpoint. Tmax is athreshold value which is set in advance by adding an allowable amount oftemporal shift from a time that represents the midpoint.

In the process illustrated in FIG. 20A, the flash emission amountcalculating section 53 compares the time Tf determined at step S508against the threshold value Tmin (step S601), and if Tf is less than thethreshold value Tmin, replaces the value of Tf stored in the internalmemory 48 with Tmin (step S602). In the case that Tf is greater than thethreshold value Tmin, Tf is compared against the threshold value Tmax(step S603). If Tf is greater than the threshold value Tmax, replacesthe value of Tf stored in the internal memory 48 with Tmax (step S604).Then, the processes of steps S601 through S604 are executed instead ofthe process of step S509, and a calculation area is determined based ona preliminary emission initiation timing which is ultimately determined(step S605).

FIG. 20B illustrates a process in which the preliminary emission periodis reset to the reference period. In the process illustrated in FIG.20B, the time Tf determined at step S508 is compared against thethreshold values Tmin and Tmax (step S611). If Tf is less than thethreshold value Tmin or greater than the threshold value Tmax, theinitiation timing of preliminary emission is set to the midpoint of aperiod beginning at a charge accumulation initiation time of line 1 anda charge accumulation completion time of line N, regardless of the areset by the user operations (step S612). Thereafter, a calculation areais determined based on the determined preliminary emission timing (stepS613).

In the embodiments illustrated in FIG. 20A and FIG. 20B, the timing ofpreliminary emission is reset in cases that the preliminary emissionperiod set to reflect user intentions is shifted greatly from a standardperiod, and there is a possibility that problems will occur in thecalculations of the flash emission amount. Therefore, appropriatecontrol can be performed, even in case that users specify inappropriateareas.

Other Embodiments

In the first through fourth embodiments described above, the facedetecting section 55, the distance measuring section 56, the AE/AWBprocessing section 46 and the image processing section 47 function asthe main region determining means, respectively. Alternatively, all ofthe face detecting section 55, the distance measuring section 56, theAE/AWB processing section 46 and the image processing section 47 mayperform processes to determine the main region, and the flash emissionamount calculating section 53 may select a main region from among thoserepresented by main region data output by each processing sectionaccording to a photography mode setting or a flash mode setting.Further, whether setting of preliminary emission timing is to beperformed may be defined for each photography mode or each flash mode,and the flash emission amount calculating section 53 may refer to themode setting and perform adjustment of the preliminary emission periodonly when necessary.

In addition, area guides similar to the area guide 11 of the fourthembodiment may also be displayed on the monitor 9 in the first throughthird embodiments. In the first through third embodiments, users cannotset areas, but by enabling users to confirm which areas are set ascalculation areas, the users can be provided with opportunities toperform photography operations again.

Note that methods for extracting pixel signals to be utilized incalculations, formulas for calculating flash emission amounts,parameters which are utilized in the calculations, and contents ofdisplay by the monitor other than those exemplified in the presentspecification are possible. Such modifications are also within thetechnical scope of the present invention.

1. An imaging apparatus, comprising: an image sensor constituted by aplurality of photoelectric converting elements which are arranged in amatrix and capable of being accessed randomly; control means forcontrolling the charge accumulation initiation timing and the chargeaccumulation cessation timing of the photoelectric converting elementsline by line or pixel by pixel; image forming means for forming imagesemploying pixel signals which are output from the photoelectricconverting elements; main region determining means for determiningportions of the images which are formed by the image forming means asmain regions; flash driving means for driving a flash during a setpreliminary emission period and a set main emission period; and periodsetting means for setting the preliminary emission period such thatcharge accumulation is initiated prior to the preliminary emissionperiod and charge accumulation is ceased after the preliminary emissionperiod in the photoelectric converting elements corresponding to pixelsthat constitute the determined main portions.
 2. An imaging apparatus asdefined in claim 1, wherein: the main region determining means detectspredetermined patterns within the images formed by the image formingmeans, and determines regions in which the predetermined patterns arepresent as the main regions.
 3. An imaging apparatus as defined in claim2, wherein: the predetermined patterns are images of faces.
 4. Animaging apparatus as defined in claim 2, wherein: the predeterminedpatterns are reference patterns which are utilized to detect movement ofsubjects.
 5. An imaging apparatus as defined in claim 1, wherein: themain region determining means divides the images formed by the imageforming means into a plurality of blocks, calculates distances from eachblock to subjects within the images, and determines regions constitutedby at least one block having a distance to the subjects less than orequal to a predetermined threshold value as the main regions.
 6. Animaging apparatus as defined in claim 1, wherein: the main regiondetermining means divides the images formed by the image forming meansinto a plurality of blocks, calculates the brightness of subjects withineach block, and determines regions constituted by at least one blockhaving a subject brightness less than or equal to a predeterminedthreshold value as the main regions.
 7. An imaging apparatus as definedin claim 1, wherein: the main region determining means receives input ofregion setting operations by a user within the images formed by theimage forming means, and determines regions set by the region settingoperations as the main regions.
 8. An imaging apparatus as defined inclaim 1, further comprising: display control means for outputting theimages formed by the image forming means and markers that indicate themain regions determined by the main region determining means to apredetermined screen.
 9. An imaging apparatus as defined in claim 1,wherein: the period setting means resets the preliminary emission periodsuch that a period which is shifted for an amount of time correspondingto a predetermined threshold value from a reference period becomes thepreliminary emission period, in the case that the amount of temporalshift between the preliminary emission period, which has been set basedon the main region, and the reference period exceeds the predeterminedthreshold value.
 10. An imaging apparatus as defined in claim 1,wherein: the period setting means resets the preliminary emission periodsuch that a reference period becomes the preliminary emission period, inthe case that the amount of temporal shift between the preliminaryemission period, which has been set based on the main region, and thereference period exceeds a predetermined threshold value.
 11. An imagingapparatus as defined in claim 1, wherein: the main region determiningmeans operates selectively based on an operating mode set in the imagingapparatus.
 12. An imaging apparatus as defined in claim 1, furthercomprising: means for recording the images formed by the image formingmeans into a predetermined recording medium, and for recording data thatspecifies the preliminary emission period as data attached to theimages.
 13. A method for controlling flash emission of an imagingapparatus that has a flash photography function, comprising the stepsof: controlling the charge accumulation initiation timing and the chargeaccumulation cessation timing of photoelectric converting elements of animage sensor constituted by a plurality of photoelectric convertingelements which are arranged in a matrix and capable of being accessedrandomly line by line or pixel by pixel; forming images employing pixelsignals which are output from the photoelectric converting elements;determining portions of the formed images as main regions; setting apreliminary emission period such that charge accumulation is initiatedprior to the preliminary emission period and charge accumulation isceased after the preliminary emission period in the photoelectricconverting elements corresponding to pixels that constitute thedetermined main portions; and driving a flash during the set preliminaryemission period.